Could a C++ implementation, in theory, parallelise the evaluation of two function arguments?

Asked 2012-11-18T19:08:43.353

Given the following function call:

f(g(), h())

since the order of evaluation of function arguments is unspecified (still the case in C++11 as far as I'm aware), could an implementation theoretically execute g() and h() in parallel?

Such a parallelisation could only kick in were g and h known to be fairly trivial (in the most obvious case, accessing only data local to their bodies) so as not to introduce concurrency issues but, beyond that restriction I can't see anything to prohibit it.

So, does the standard allow it? Even if only by the as-if rule?

_{(In this answer, Mankarse asserts otherwise; however, he does not cite the standard, and my read-through of [expr.call] hasn't revealed any obvious wording.)}

c++ language-lawyer

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2 Answers

As long as you can't tell, whatever the compiler does to evaluate these functions is entirely up to the compiler. Clearly, the evaluation of the functions cannot involve any access to shared, mutable data as this would introduce data races. The basic guiding principle is the "as if"-rule and the fundamental observable operations, i.e., access to volatile data, I/O operations, access to atomic data, etc. The relevant section is 1.9 [intro.execution].

answered 2012-11-18T19:25:45.427

Not unless the compiler knew exactly what g(), h(), and anything they call does.

The two expressions are function calls, which may have unknown side effects. Therefore, parallelizing them could cause a data-race on those side effects. Since the C++ standard does not allow argument evaluation to cause a data-race on any side effects of the expressions, the compiler can only parallelize them if it knows that no such data race is possible.

That means walking though each function and look at exactly what they do and/or call, then tracking through those functions, etc. In the general case, it's not feasible.

answered 2012-11-18T19:24:45.813

Could a C++ implementation, in theory, parallelise the evaluation of two function arguments?

2 Answers

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